The present invention relates to carrier wave intelligence transmission systems in general and, more particularly, to a method and apparatus for transmitting intelligence over electric power distribution networks.
The use of electric power lines for signalling, meter reading, load control, and other communication purposes is well known in the art. Representative examples of system which utilize power lines for the communications medium are shown in the following U.S. Pat. Nos: 1,888,555, Nov. 22, 1932 to A. Hund for System of Electrical Distribution; 2,001,450, May 14, 1935 to C. A. Boddie for Transmitting Circuit; 2,263,389, Nov. 18, 1941 to W. Koenig for Remote Control System; 2,457,607, Dec. 28, 1948 to C. E. Seymour for Remote Control System; 2,494,873, Jan. 17, 1950 to W. C. Hall for Remote Control Unit; 2,580,539, Jan. 1, 1952 to C. L. Goodwin for Electrical Remote-Control System; 2,745,991, May 15, 1956 to C. E. Seymour for Centralized Load Control Systems; 2,860,324 Nov. 11, 1958 to F. C. E. M. Berger et al for Power Line Signalling System; 2,942,243, June 21, 1960 to W. Bilz for Automatic Recording System; 2,962,702, Nov. 29, 1960 to W. A. Derr et al for Remote Metering; 2,972,686, Feb. 21, 1961 to J. C. G. Pelpel for Remote Control System for Lattice Distribution Network; 3,011,102, Nov. 28, 1961 to I. Balan for Control System; 3,058,095, Oct. 9, 1962 to A. C. Reynolds, Jr. for Binary Code Relay; 3,067,405, Dec. 4, 1962 to H. Hurlimann et al for Method of Remotely Controlling Electric Switching Arrangements By Means of Mains-Superposition Central Remote Control Installations And Arrangement For Carrying Out The Method; 3,098,215, July 16, 1963 to D. P. Waite for Data Storage And Transmission System; 3,121,859, Feb. 18, 1964 to W. E. Furniss for Remote Metering System; 3,164,771, Jan. 5, 1965 to R. E. Milford for Apparatus For Central Recording Of Remote Meter Data By Periodic And Sequential Meter Interrogation; b 3,229,300; Jan. 11, 1966 to R. J. Thompson et al for Data Gathering And Recording System; 3,234,543, Feb. 8, 1966 to R. J. Thompson et al for Carrier Current Transmitter Unit for Electrically Powered Devices; 3,258,692, June 28, 1966 to O. J. Jacomini et al for Automatic Reading Apparatus For Plural Meters By Transmitted Coded Pulse Trains; 3,264,633, Aug. 2, 1966 to M. W. Hellar For Automatic Power Meter Reading Over Neutral Power Transmission Line; 3,388,388, June 11, 1968 to R. A. Brown for Cumulative Digital Pulse Remote Meter Reading; 3,359,511, Dec. 19, 1967 to R. L. Dennison for System For Remotely Controlling The Operation of A Power Distribution Network; 3,445,814, May 20, 1969 to A. Spalti for System For Interrogating Remote Stations Via Power Lines of An Electrical Distribution Network; 3,454,910, July 8, 1969 to A. Nyfeler for Vibratory Switching Mechanism; 3,458,657, July 29, 1969 to R. W. Lester et al for Remote Control Over Power Lines By Transmitting High Frequency Pulses In Phase With Positive And Negative Half Cycles Of The Power Line Current; 3,460,121, Aug. 5, 1969 to W. H. Wattenburg for Signalling And Communication System; 3,462,756, Aug. 19, 1969 to G. A. Mills for Apparatus For Transmitting And Receiving A High Frequency Transient Over A Power Line; 3,482,243, Dec. 2, 1969 to W. H. Buchsbaum for Protective System; 3,483,546, Dec. 9, 1969 to R. A. Ausfeld for Power Line Communication Systems; 3,484,694, Dec. 16, 1969 to A. Brothman et al for Data Transmission System Wherein System Control is Divided Between A Plurality Of Levels For Remote Location Activation; 3,488,517, Jan. 6, 1970 to J. M. Cowan et al for Control Systems; 3,503,044, Mar. 24, 1970 to P. I. Bonyhard et al for Magnetic Domain Shift Register Meter Reader; 3,508,243, Apr. 21, 1970 to A. Nyfeler et al for Telemetry Arrangements Utilizing Power Distributing Networks For Measuring Consumption; 3,509,537, Apr. 28, 1970 to C. F. Haberly for Digital Communication Apparatus For A Power Distribution Line; 3,540,030, Nov. 10. 1970 to E. M. Hartz for Structure For And Method Of Powerline Load Remote Control; 3,559,177, Jan. 26, 1971 to R. A. Benson for Variable Length, Diverse Format Digital Information Transfer System; 3,594,584, July 20, 1971 to R. E. Woods for Telemetry Circuit For An AC Power System; 3,626,297, Dec. 7, 1971 to S. A. Green for Transfer Trip System Using Quadrature Carrier Modulation With Coherent Detection; 3,626,369, Dec. 7, 1971 to J. D. Ainsworth for Telecommunication Control System; 3,653,024, Mar. 28, 1972 to C. E. G. Lundgren et al for Method Of Selected Diverging Communication And The Combination Thereof With Selected Diverging Communication Between A Group Of Population To A Remote Center, And A System For Carrying Out The Method; 3,654,605, Apr. 4, 1972 to Y. Honda et al for Remote Meter Reading System Having Electro-Mechanical Oscillators; 3,656,112, Apr. 11, 1972 to S. Paull for Utility Meter Remote Automatic Reading System; 3,662,366, May 9, 1972 to C. M. D. Neuville et al for Process For The Remote Reading Of Members For Detecting Various Variables, Particularly Of Meters And Similar, And Device For Operating The Same; 3,683,343, Aug. 8, 1972 to S. Feldman et al for Demand Metering System For Electric Energy; 3,702,460, Nov. 7, 1972 to J. B. Blose for Communications System For Electric Power Utility; 3,710,373, Jan. 9, 1973 to S. Watanabe et al for Signal Discriminating System; 3,714,451, Jan. 30, 1973 to J. A. Whitney et al for Phase Selective Telemetry System; 3,719,928, Mar. 6, 1973 to H. Oishi et al for Sweep Signal Meter Reading System; 3,721,830, Mar. 20, 1973 to H. Oishi et al for Pulse Dip Carrier System Using AC Distribution Line; 3,733,586, May 15, 1973 to J. F. Lusk et al for Meter Interrogation System Having Strobe Logic Control. Other combined power line and signalling communication systems are found in Class 340, Subclass 310.
In any discussion of power line communication techniques, a distinction should be made between electric transmission lines and electric distribution systems. Electric transmission lines exhibit relatively little branching. Accordingly, communication at carrier frequencies in the range 100 to 300 kHz has proved feasible by the simple expediency of providing appropriate filtering and by-pass circuitry at the various junctions. Transmission lines tend to be straight over relatively long distances and in recent years microwave links have tended to replace the earlier carrier schemes.
The electric distribution system differs markedly from transmission lines. Each customer service constitues a branch in the distribution feeder which follows a circuitous path so as to pass in close proximity to the customer's premises. The branching is so extensive that it is impractical to provide filter and by-pass circuitry at each branch point, thus the techniques of carrier communication used on transmission lines are inappropriate for communication in the distribution system. Furthermore, the tortuous path of the distribution feeder precludes the use of microwaves.
There is a growing recognition of the need for communication preferable, digital over the distribution feeders. In support of this need are the following considerations.
1. At the present time utilities are striving to improve load factors as a means of decreasing peak demand and thereby reducing the need for peak generating units which use hydrocarbon fuels. One approach for improving load factors is the automatic control of loads presented by domestic water heaters. These loads are expected to account for 6.5% of the total electric energy consumption in the United States by 1976.
2. Higher voltage feeders have been introduced to satisfy the demand for increased power. The rise in feeder voltage has produced a concomitant increase in the vulnerability of the feeder to faults. When feeders were supplied at lower voltage, fault isolation equipment was customarily located only at the substation. If the higher voltage feeders are to provide a quality of service comparable to that provided from lower voltage lines, it is important that rapid means be provided for isolation of faulted sections of feeders and for restoration of service to unfaulted sections.
3. There is also a requirement for the controlled switching of the power-factor-correcting capacitors which are used to compensate for the lagging phase angle that results from the customers' inductive loads. The management of utility power systems can be better accomplished by means of controlled switching of capacitor banks in preference to automated switching by means of time clocks, voltage sensors, etc.
4. Automatic meter reading is attracting increased attention. Manual meter reading is highly labor-intensive and accordingly its costs are rising steeply. When access to meters is impossible, billings are made on the basis of estimated readings and such estimated billings often lead to customer complaints.
5. There is an interest in monitoring conditions and events at sites served by electric power. For example, monitoring functions can include the following:
A. In oil fields--turning pumps "off" or "on", reading volume flow, checking on operating state of pumps and motors;
B. Alarm systems--intrusion alarms, low temperature alarms, smoke and fire, etc.
The distribution system is not an attractive medium for conventional communications due to attenuation and dispersion of the signals and because the noise levels tend to be high. To overcome the high noise levels it is generally necessary to use narrow band filtering, error-detecting and error-correcting codes, and relatively high signalling power level at low bit rates.
Recent investigations relating to communication in the distribution system include the following:
1. Automated Technology Corporation has used three level frequency shift keying in the frequency range which passes through distribution transformers by virtue of the interwinding capacitance. However, these signals are very severely attenuated on passing through distribution transformers and on passing power-factor-correcting capacitors.
2. General Public Utilities has employed two level frequency shift keying with frequencies in the range 900 Hz to 1100 Hz (See U.S. Pat. No. 3,733,586). These signals pass through distribution transformers by inductive coupling. However, power-factor-correcting capacitors introduce troublesome attenuation and signal attenuation is markedly greater during periods of peak demand than during off-peak periods.
3. A broad range of frequencies is being examined by a number of workers at the present time with particular attention centered on the frequencies in the range of 100 to 300 kHz and in the upper audio spectrum.
4. Various European companies currently offer "ripple control" systems in which frequencies ranging from 140 Hz to 750 Hz are superimposed on the transmitted power. Different frequencies are used to prevent cross-talk between interconnected utility systems. Each coded transmission lasts for about 30 seconds.
The above signalling approaches generally suffer from one or more deficiencies. For example, the power levels used in the "ripple control" approach and in other signalling techniques, are high and costs of such signal generation and signal detection are considered by many to be excessive. The use of narrow band transmission requires that the transmitter and detector employ the same technique for establishing the frequency window. Although this can be accomplished either through phase-locked loops which are governed by line frequency or by crystal control, such techniques introduce additional circuit complexity and cost.
It is accordingly a general object of the invention to provide an improved method and apparatus for transmitting intelligence over a carrier wave so as to preserve undisturbed the usual useful characteristics of the carrier wave while producing recognizable changes in its characteristics which will permit its use for transmission of additional data;
It is a specific object of the invention to provide an improved method and apparatus for transmitting intelligence over electric power lines;
It is another object of the invention to provide a method for transmitting intelligence over electric power lines utilizing existing hardware components.
It is a feature of the invention that power-factor correcting capacitors and transformers introduce no appreciable signal attenuation.
It is another feature of the invention that the method thereof can be practiced with relatively inexpensive hardware without impairing the reliability of the communications system.